Magnetic tape recorders, such as telephone answering devices, commonly use magnetic tapes on which analog signals are recorded. Such devices require a circuit which adds a bias signal to the recorded analog signal so that the dynamic range of the magnetic tape may be optimized. It is also necessary for such devices to incorporate additional circuits which are used to erase signals from the magnetic tapes so that new signals can be recorded.
In telephone answering devices and other low-cost recorders, the bias signal is normally provided by adding a DC voltage to the analog signal. A circuit for realizing this biasing method is shown schematically in FIG. 1. In FIG. 1 a record/playback head 10 is connected to input amplifier 14, via switch 12, and to output amplifier 16. When switch 12 is open, the device is in playback mode (i.e., the signals on the tape are reproduced) and an output signal is provided by the output amplifier 16. When switch 12 is closed, the record/playback head 10 is in record mode to record the analog signal on the tape. In this mode the record head 10 receives the analog input signal from amplifier 14. A DC voltage from DC bias source 18 is combined with the analog input signal by an adder circuit 20. This combined signal is provided to the record/playback head 10 via input amplifier 14.
In order to erase the tape, the DC voltage from the DC bias source 18 is also received by an erase head 24 via an erase amplifier 22. A switch 26 is provided in order to disable erasing when the device is not intended to erase.
This biasing/erasing system is inexpensive and reliable and does not require careful adjustment or extensive maintenance. However, as is well-known, the DC method of biasing uses only part of the magnetization curve of the recording head 10. Consequently, the system typically provides a signal-to-noise (S/N) ratio of less than 30 decibels (dB). A careful investigation has revealed that the principle noise contributor in such a system is the DC bias arrangement itself.
By contrast, in high quality, professional, magnetic tape recorders, a biasing method known as AC biasing is used. A circuit embodying this method is shown in FIG. 2. The AC biasing method involves adding a large amplitude, fixed, supersonic, single frequency sinusoidal signal to the analog signal before coupling the signal to the record head 10. (Similar reference numbers in FIGS. 1 and 2 indicate similar structures.) In place of the DC bias source 18 (FIG. 1), the AC biasing method uses an AC bias oscillator 30. This oscillator provides a second bias signal to an amplitude control circuit 32. The amplitude control circuit 32 subsequently provides an AC biasing signal to the adder circuit 20 where the bias signal is combined with the analog signal to be recorded. The combined signal is provided, in turn, to an amplifier 14 and, via switch 12 to the record head 10 and output amplifier 16.
Oscillator 30 also provides a AC erase signal to the erase head 24 via a step up transformer 31 and a switch 26. The step up transformer is necessary to increase the erase signal voltage so that it completely saturates the tape in order to erase it completely. Due to the presence of the transformer, the AC signal provided to erase head 24 usually has a frequency equal to the frequency provided to bias the record head 10 or equal to an odd submultiple of the bias frequency.
The AC biasing/erasing arrangement is capable of producing superior sound quality when compared to the DC bias/erase arrangement discussed above--with such AC systems, S/N ratios of 60 to 70 dB can be achieved. However, during initial factory set up, the frequency of the AC bias oscillator 30 (typically 30 to 40 kHz), and the RMS value of the AC bias waveform (typically about 50 volts) must be carefully adjusted to maintain such high S/N ratios. In order to reduce distortion further, the oscillators must also be fine tuned so that the output waveform is symmetrical. In addition, the circuitry required to control the frequency and amplitude of the AC bias waveform is costly. Further, the step up transformer 31 contributes significantly to the cost of the erase circuit. Consequently, the conventional AC bias/erase circuit is generally too costly for relatively low cost magnetic tape recorders such as telephone answering devices and is only cost effective for expensive devices, such as professional quality magnetic tape recorders.
Accordingly, it is an object of the present invention to provide a biasing/erasing method and apparatus for a magnetic tape recording apparatus, especially telephone answering and other low cost recording devices, which method and apparatus improve the signal to noise (S/N) ratio over a conventional DC biasing/erasing system.
It is another object of the present invention to provide a biasing method and apparatus which improve the signal to noise (S/N) ratio over a conventional DC biasing/erasing system without greatly increasing the cost.
It is yet another object of the present invention to provide a biasing method and apparatus which utilize AC biasing for a high S/N ratio and which eliminate the need for a step up transformer in the erase circuit.